1. Field of the Invention
The present invention relates to a process for thermally fixing a latent electrostatic image which has been rendered visible by means of a suspension developer on a support by applying heat and vaporizing the developing liquid. The invention also relates to an apparatus which is suitable for carrying out the process.
2. Discussion of Related Art
Thermal fixing is generally applied to latent electrostatic images which have been rendered visible. These so-called toner images, which are generated by liquid development of charge images of various origin, are, for example, produced in electrostatic printers with the aid of recording electrodes, by electron beam recording, X-ray recording in ionization chambers or, as widely used, by charging and exposing photoconductive layers. Processes are known in which the toner image prepared is, for example, fixed on a photoconductive layer, as in the case of zinc-oxide coated papers or electrophotographic printing plates, or in which the toner image is transferred from an original support, for example, a photoconductive layer, to a copy support, consisting of paper or film, on which it is then fixed. Many conventional office copiers use the last-mentioned principle. Thermal fixing which is equivalent to drying moist copies is, for example, also employed under ecologically acceptable conditions in the so-called ink-jet recording, where an ink jet which is controlled and modulated according to the information received, is directed to a support.
The electrophotographic copying technology used in office copiers has today gained great importance and has become the preferred, although not the exclusive field of application for thermal fixing. Without intending to limit the present invention, the description of the invention is, therefore, based on this technology.
The essential component in known electrophotographic office copiers run with liquid development, is the photoconductive medium which has the form of a plate, a web or, usually a drum. In the present case, the specific construction of the photoconductive medium comprising organic or inorganic materials or one or several optionally insulating layers is not significant. Generally, the photoconductive layer is first electrostatically charged by a corona and then imagewise exposed. By applying a pigmented powder, a so-called dry toner, or a liquid toner the resulting latent electrostatic image is rendered visible, for example, with the aid of a developing electrode. Usually, the suspension developer is composed of a dispersing liquid, preferably an aliphatic hydrocarbon, and various additions, among others a finely divided charged pigment. In most copiers, the layer thickness of the liquid developer on the photoconductor is reduced, for example, by a metering roll or by spraying-on corona charges. For transferring the toner image from the photoconductor to the copy support, for example paper, the copy support is moved to the photoconductor and to assist transfer electrostatically, the copy support is charged by a corona on the reverse side. In the transfer procedure, the copy support picks up pigment in imagewise distribution and, over its entire surface, also a large portion of the dispersing liquid. Only after evaporation of the developing liquid, will the toner image become fast and resist wiping. To make the toner image fast, the copy is warmed up, for example, by means of a heater plate. By a stream of air from a fan which is mounted in a suitable position in the copier and which, for example, also serves to cool the exposure lamps, the vaporized developing liquid is expelled through slots provided in the copier housing. The dried copy is then placed on a platform. Before the next copying cycle, the photoconductive surface is freed from any toner and liquid residue and from residual charges, for example, with the aid of a foam plastic roller equipped with an elastic wiper and by the action of an AC corona.
It is a disadvantage of the above-described copying technique run with liquid development that the developing liquid vaporized from the copy is discharged to the environment. Although the developing liquid, for example, an aliphatic hydrocarbon, is not toxic as such, the emission of vaporized liquid to the surroundings, nevertheless, causes an undesirable burden on the environment.